To obtain a desired postoperative refractive outcome of an intraocular lens implantation--emmetropia or ametropia--, there are several methods in use to determine which dioptric power the intraocular lens to be implanted, should have. The correct implant power to choose depends on the axial distance from the cornea, at which the intraocular lens (IOL) will end up in the eye.
With the present techniques, the axial position of the IOL can only be estimated.
Two major schools exist today for estimating the axial position of the IOL.
One school describes the optics of the eye in terms of thin lens theory.
In this connection reference is hereby made to:
1) Fedorov S N, Kolinko A L. Estimation of optical power of the intraocular lens. Vestn. Oftamol 1967;80(4):27-31, PA0 2) Colenbrander M C. Calculation of the power of an iris clip lens for distant vision. Br J Ophthalmol 1973;57:735-740, PA0 3) Hoffer K J. Mathematics and computers in intraocular lens calculation. Am Intra-Ocular Implant Soc J 1975;1(1):4-5, PA0 4) van der Heijde G L. A nomogram for calculating the power of the prepupillary lens in the aphakic eye. Bibliotheca Ophthalmol 1975;83:273-275, PA0 5) Thijssen J M. The emmetropic and the iseikonic implant lens: computer calculation of the refractive power and its accuracy. Ophthalmologica 1975;171:467-486, PA0 6) Binkhorst R D. The optical design of intraocular lens implants. Ophthalmic Surg 1975;6(3):17-31, and PA0 7) Holladay J T, Prager T C, Chandler T Y, et al. A three-part system for refining intraocular lens power calculations. J Cataract Refract Surg 1988;14:17-24. PA0 8) Sanders D R, Retzlaff J, Kraff M, et al.: Comparison of the accuracy of the Binkhorst, Colenbrander, and SRK.TM. implant power prediction formulas. Am Intra-Ocular Implant Soc J 1981; 7:337-340, PA0 9) Sanders D R, Retzlaff J, Kraff M C. Comparison of the SRKII.TM. formula and other second generation formulas. J. Cataract Refract Surg 1988;14:136-141, and PA0 10) Sanders D R, Retzlaff J, Kraff M C, Gimbel H V, Raanan M G. Comparison of the SRK/T formula and other theoretical and regression formulas. J Cataract Refract Surg 1990; 16:341-346. PA0 11) Olsen T. Theoretical approach to intraocular lens calculation using Gaussian optics. J Cataract Refract Surg 1987; 13:141-145, PA0 12) Haigis W. Strahldurchrechnung in Gausscher Optic zur Beschreibung des Linsen-Systems Brille-Kontaktlinse-Hornhaut-Augenlinse (IOL), in: Schott K, Jacobi K W, Freyler H (Hrsg): 4 Kongr. d. Deutsch. Ges. f. Intraokularlinsen Implant., Essen 1990. Berlin, Heidelberg, New York, Springer Verlag 1990, and PA0 13) Kashiwagi T. Ray tracing error correction in ophthalmic optics. J Cataract Refract Surg 1991; 17:194-198, PA0 a) determining the location of the lens haptic plane of the eye, PA0 b) determining the corneal power of the eye, PA0 c) determining the axial length of the eye, PA0 d) choosing the desired postoperative refraction, PA0 e) assuming a lens to be implanted, said lens having a known power and geometry, including the offset between the haptic plane of said lens and the anterior vertex of said lens as if it was in its implanted state, PA0 f) calculating from the parameters given by a), b), c), d) and e), as well as the refractive indices of ocular fluids, whether or not, postoperatively, focus will fall on the retina of the eye, PA0 g) if that is not the case, repeating steps d)-f) assuming another lens with a different power and/or geometry, until focusing on the retina is calculated in step f), and PA0 h) selecting for implantation, the lens of the nearest power available for which focusing on the retina is calculated in step f).
The axial position of the IOL is mostly considered to be a constant, often referred to as the ACD constant. The value of the constant depends to some extent on the IOL model. In the thin lens theory, this constant represents the postoperative distance between the principal planes of the cornea and of the IOL.
Another school applies retrospective statistical analysis of clinical data to determine a coefficient, the so called A-constant, in a linear equation, known as the SRK formula, linking corneal dioptric power K, eye length L, IOL power and postoperative refraction.
In this connection reference is made to:
The linear relationship mentioned above, is not a theoretically correct representation of the optics of the eye, but the SRK approach is most widely used because it is simple and, in clinical practice, yields results similar to the thin lens theory approach.
The SRK/T formula in the above reference 10), is a hybrid between the two approaches.
In both schools, further refinement entails corrections depending on mainly eye length.
The following references:
apply thick lens theory, which is physically more exact, but the general problem of pre-estimating the axial position of the IOL remains.
The position of the IOL optic is determined by its fixation in the eye. Fixation is mostly obtained by means of attachments to the optic, so called loops, that hold the lens in place by spring action against ocular tissue. The most common site of placement of the IOL today is inside the capsular bag. Alternative placements are in the ciliary sulcus and in the anterior chamber angle. Lenses that are fixed to the iris also exist, but fixation is then not by spring action. There are also lenses meant for capsular bag placement that do not possess loops or exert spring action, such as disc lenses, plate lenses, and capsular bag filling lenses.